diff --git a/Assets/VRM/UniJSON/Editor/Tests/Json/ValidatorTests.cs b/Assets/VRM/UniJSON/Editor/Tests/Json/ValidatorTests.cs
index d6a475251..7165a3d37 100644
--- a/Assets/VRM/UniJSON/Editor/Tests/Json/ValidatorTests.cs
+++ b/Assets/VRM/UniJSON/Editor/Tests/Json/ValidatorTests.cs
@@ -199,6 +199,41 @@ namespace UniJSON
             Assert.True(c.IsEmpty());
         }
 
+        class NotRequired
+        {
+            [JsonSchema(Minimum = 1)]
+            public int Value;
+        }
+
+        [Test]
+        public void ObjectValidatorForNotRequired()
+        {
+            {
+                var c = new JsonSchemaValidationContext("test")
+                {
+                    EnableDiagnosisForNotRequiredFields = false, // Default behaviour
+                };
+
+                var s = JsonSchema.FromType<NotRequired>();
+                // An error is not returned because Value is not 'Required' and the diagnosis is not enabled
+                Assert.Null(s.Validator.Validate(c, new Hoge { Value = 0 }));
+
+                Assert.True(c.IsEmpty());
+            }
+
+            {
+                var c = new JsonSchemaValidationContext("test")
+                {
+                    EnableDiagnosisForNotRequiredFields = true,
+                };
+
+                var s = JsonSchema.FromType<NotRequired>();
+                Assert.NotNull(s.Validator.Validate(c, new Hoge { Value = 0 }));
+
+                Assert.True(c.IsEmpty());
+            }
+        }
+
         [Test]
         public void DictionaryValidator()
         {
diff --git a/Assets/VRM/UniJSON/Scripts/JsonSchemaValidator/IJsonSchemaValidator.cs b/Assets/VRM/UniJSON/Scripts/JsonSchemaValidator/IJsonSchemaValidator.cs
index 6c500e0d3..9a53769d8 100644
--- a/Assets/VRM/UniJSON/Scripts/JsonSchemaValidator/IJsonSchemaValidator.cs
+++ b/Assets/VRM/UniJSON/Scripts/JsonSchemaValidator/IJsonSchemaValidator.cs
@@ -7,6 +7,8 @@ namespace UniJSON
     {
         Stack<string> m_stack = new Stack<string>();
 
+        public bool EnableDiagnosisForNotRequiredFields = false;
+
         public JsonSchemaValidationContext(object o)
         {
             Push(o.GetType().Name);
diff --git a/Assets/VRM/UniJSON/Scripts/JsonSchemaValidator/JsonObjectValidator.cs b/Assets/VRM/UniJSON/Scripts/JsonSchemaValidator/JsonObjectValidator.cs
index ae943ae65..a4dd1e8e6 100644
--- a/Assets/VRM/UniJSON/Scripts/JsonSchemaValidator/JsonObjectValidator.cs
+++ b/Assets/VRM/UniJSON/Scripts/JsonSchemaValidator/JsonObjectValidator.cs
@@ -28,11 +28,11 @@ namespace UniJSON
             get; set;
         }
 
-        List<string> m_required = new List<string>();
+        HashSet<string> m_required = new HashSet<string>();
         /// <summary>
         /// http://json-schema.org/latest/json-schema-validation.html#rfc.section.6.5.3
         /// </summary>
-        public List<string> Required
+        public HashSet<string> Required
         {
             get { return m_required; }
         }
@@ -302,7 +302,7 @@ namespace UniJSON
         {
             class ObjectValidator
             {
-                delegate JsonSchemaValidationException FieldValidator(IJsonSchemaValidator v, 
+                delegate JsonSchemaValidationException FieldValidator(IJsonSchemaValidator v,
                     JsonSchemaValidationContext c, T o);
 
                 Dictionary<string, FieldValidator> m_validators = new Dictionary<string, FieldValidator>();
@@ -341,32 +341,38 @@ namespace UniJSON
                     }
                 }
 
-                public JsonSchemaValidationException Validate(List<string> required, Dictionary<string, JsonSchema> properties,
+                public JsonSchemaValidationException Validate(
+                    HashSet<string> required,
+                    Dictionary<string, JsonSchema> properties,
                     JsonSchemaValidationContext c, T o)
                 {
-                    foreach (var x in required)
+                    foreach (var kv in properties)
                     {
-                        JsonSchema s;
-                        if(properties.TryGetValue(x, out s))
+                        var fieldName = kv.Key;
+                        var schema = kv.Value;
+
+                        FieldValidator fv;
+                        if (m_validators.TryGetValue(fieldName, out fv))
                         {
-                            FieldValidator fv;
-                            if (m_validators.TryGetValue(x, out fv))
+                            var ex = fv(schema.Validator, c, o);
+                            if (ex != null)
                             {
-                                var ex = fv(s.Validator, c, o);
-                                if (ex != null)
+                                if (required.Contains(fieldName) // required fields must be checked
+                                || c.EnableDiagnosisForNotRequiredFields)
                                 {
                                     return ex;
                                 }
                             }
                         }
                     }
+
                     return null;
                 }
             }
 
             static ObjectValidator s_validator;
 
-            public static JsonSchemaValidationException Validate(List<string> required,
+            public static JsonSchemaValidationException Validate(HashSet<string> required,
                 Dictionary<string, JsonSchema> properties,
                 JsonSchemaValidationContext c, T o)
             {
@@ -390,16 +396,7 @@ namespace UniJSON
                 return new JsonSchemaValidationException(c, "no properties");
             }
 
-            if (Required != null)
-            {
-                var ex = GenericValidator<T>.Validate(Required, Properties, c, o);
-                if (ex != null)
-                {
-                    return ex;
-                }
-            }
-
-            return null;
+            return GenericValidator<T>.Validate(Required, Properties, c, o);
         }
 
         static class GenericSerializer<T>
@@ -511,7 +508,7 @@ namespace UniJSON
             GenericSerializer<T>.Serialize(this, f, c, value);
         }
 
-        static class GenericDeserializer<S, T> 
+        static class GenericDeserializer<S, T>
             where S : IListTreeItem, IValue<S>
         {
             delegate T Deserializer(ListTreeNode<S> src);
@@ -596,7 +593,7 @@ namespace UniJSON
             }
         }
 
-        public void Deserialize<T, U>(ListTreeNode<T> src, ref U dst) 
+        public void Deserialize<T, U>(ListTreeNode<T> src, ref U dst)
             where T : IListTreeItem, IValue<T>
         {
             GenericDeserializer<T, U>.Deserialize(src, ref dst, Properties);